The invention relates to an integrated process for the preparation of epoxidised olefins. In the first stage, dilute hydrogen peroxide solutions are prepared from the elements hydrogen and oxygen, with catalysis, and are reacted in a subsequent liquid-phase epoxidation with olefin in the presence of titanium silicalite to form epoxidised olefin, and the solvents are returned to the H2O2 process again.
Propene oxide is one of the most important basic chemicals in the chemical industry. Its field of application, where over 60% is used, is the plastics sector, especially in the preparation of polyether polyols for the synthesis of polyurethanes. In addition, the propene oxide derivatives also make up relatively large shares of the market in the field of the glycols, especially lubricants and antifreeze.
The preparation of epoxidised olefins is known in principle. An olefin is used as the starting material and is oxidised by a very wide variety of methods. For ecological reasons, however, oxidation using hydrogen peroxide or air is to be preferred.
Titanium silicalite-catalysed epoxidation using pure oxygen as the oxidising agent is possible in the presence of a redox system consisting of alkylanthrahydroquinone and alkylanthraquinone (U.S. Pat. No. 5,221,795). A disadvantage of this reaction is the continuous loss of small amounts of anthraquinone and organic solvents as a result of oxidative decomposition of those organic compounds.
Using titanium silicalites containing platinum metal, propene oxidation is possible with a low yield (approximately from 1 to 2%) and with unsatisfactory propene oxide selectivities of from 60 to 70% using a gas mixture consisting of molecular oxygen and molecular hydrogen (WO-97/47 386, WO-96/023 023). Hydrogenations occurring as a side reaction lead to large amounts of propane as a secondary product. U.S. Pat. No. 5,623,090 and WO-98/00 413-15 disclose a direct oxidation of propene to propene oxide using molecular oxygen in the presence of hydrogen. Commercially available titanium dioxide containing finely dispersed gold particles is used as the catalyst. In addition to low yields, all those processes have the disadvantage that they are very expensive owing to the gold content of the catalyst. For economical use, therefore, the development of catalysts having markedly better catalyst activities with a greatly increased useful life of the catalyst continues to be absolutely necessary.
U.S. Pat. No. 4,833,260 describes titanium silicalite catalysts which effectively permit the epoxidation of olefins using the oxidising agent hydrogen peroxide in the liquid phase. In the case of the silicalites, a small portion of the lattice silicon has been replaced by titanium (U.S. Pat. No. 4,410,501). However, the high costs of hydrogen peroxide as an oxidising agent have hitherto prevented its use on a large scale. A large part of the costs involved in the use of hydrogen peroxide arise due to the hydrogen peroxide itself, since the concentrations of hydrogen and oxygen during the preparation must be observed very closely for safety reasons. Reaction solutions containing low concentrations of H2O2 are predominantly obtained, and they must then be concentrated, purified and stabilised in costly operations.
The preparation of hydrogen peroxide is in principle state of the art.
It has for a long time been known that hydrogen peroxide can also be prepared directly from the elements hydrogen and oxygen using suitable catalysts.
It has also for a long time been known that mixtures of gaseous oxygen and hydrogen yield explosive gas mixtures. For that reason, all industrial H2O2 processes are carried out using an indirect combination of hydrogen and oxygen.
Over 90% of world hydrogen peroxide production is currently carried out by the anthraquinone process, in which alkylanthraquinones are typically used as the chemical auxiliary substances. The disadvantage of this reaction is the continuous loss of small amounts of anthraquinone and organic solvents as a result of the oxidation and of thermal decomposition of those organic compounds and cost-intensive extraction, purification and distillation steps.
The direct synthesis of H2O2 from the elements hydrogen and oxygen is the subject of intensive research efforts, but as yet has not led to any commercial application. Aside from the safety problems, the most important problem is to prevent the hydrogen peroxide that is formed from subsequently decomposing to water and oxygen. That problem is solved by means of continuous processes which operate at high rates of flow. The result is, however, that at low reaction rates, the hydrogen peroxide concentrations contained in the discharge are too low for economical use of that process.
For the formation of hydrogen peroxide from hydrogen and oxygen using palladium-containing catalysts, U.S. Pat. No. 4,009,252 discloses an optimum ratio of O2 to H2 in the range of from 1.5:1 to 20:1, i.e. in the explosive range.
The transition metals of sub-group 8 , mostly palladium or platinum, are most frequently used as the catalytically active species. The noble metal can be applied to various supports, such as TiO2, SiO2, Al2O3, Teflon, activated carbon or catalyst monoliths produced from woven fabrics, such as, for example, V4a, with activated carbon being most frequently used. Processes based on such catalyst systems have been patented by numerous firms and institutions, such as, for example, U.S. Pat. Nos. 4,279,883, 4,661,337, EP-117 306 and DE-196 42 770.
U.S. Pat. Nos. 4,336,238 and 4,336,239 describe the reaction of hydrogen and oxygen to form hydrogen peroxide using palladium-containing catalysts in organic solvents or solvent mixtures, which optionally also contain water. The hydrogen peroxide concentrations of at most 2.4 wt. % which are obtained using reaction gas mixtures containing less than 5 vol. % hydrogen are too low for economical use. Moreover, after an operating time of 285 hours, the catalyst activity has fallen to 69% of the original value, which is still too low for industrial use.
U.S. Pat. No. 5,352,645 and WO-92/04 976 describe special solid supports of spray-dried coloidal silica gel. EP-627 381 discloses the use of niobium, tantalum, molybdenum or tungsten oxides as support materials which are distinguished by high resistance to acid.
In the mentioned specifications, however, the hydrogen peroxide is always prepared by batch or semi-continuous processes, which are not very suitable for industrial use. In addition, the short reaction times allow no conclusion to be drawn regarding the useful life of the catalysts.
DE-A-196 42 770 discloses the preparation of hydrogen peroxide using palladium-containing catalyst monoliths, for example V4a nets or woven fabrics impregnated with palladium. C1-C3-alcohols, or mixtures with water, are used as the solvent. Palladium is predominantly used as the catalytically active component; suitable promoter substances are preferably noble metals, such as platinum, rhodium, gold and silver.
None of those processes has gained acceptance for conventional H2O2 installations because of the low H2O2 concentration and, in some cases, the presence of solvents.
Not a single process has hitherto been known for the preparation of propylene oxide from propylene and hydrogen peroxide, in which hydrogen peroxide solutions which have not been concentrated and have been only crudely pre-purified are used and are returned to the hydrogen peroxide preparation again after the propylene oxidation.
Surprisingly, the Applicants have found that hydrogen peroxide solutions which have not been concentrated and have been only crudely purified can be used directly from the preparation for preparing epoxidised olefins, especially propene oxide, by the catalysed epoxidation of olefins by means of H2O2 in the presence of a zeolite, containing synthetic titanium atoms, of the general formula xTiO2xc2x7(1xe2x88x92x)SiO2, wherein x is in the range of from 0.0001 to 0.04.
The aqueous or aqueous-alcoholic hydrogen peroxide solutions prepared in the first stage can, after slight purification, be reacted selectively with olefins in the presence of the said zeolites containing synthetic titanium atoms to form the epoxidised olefin. The process provides for the separation and return of solvents, so that they can be returned to the hydrogen peroxide preparation process again without additional purification.